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Science: Universe will expand forever

18 December 1993

By
KEN CROSWELL in
BERKELEY

A group of galaxies in the constellation of Cepheus contains far less
invisible matter than had been thought a few months ago, say astronomers
in the US and France. Their finding suggests that the Universe is ‘open’
– that is, it has insufficient mass for gravity to one day stop and reverse
the expansion.

Earlier this year, John Mulchaey of the Space Telescope Science Institute
in Baltimore and his colleagues reported that they had discovered hot gas
in a small collection of galaxies called the NGC 2300 group. From the properties
of this gas, the astronomers deduced that the total mass in the NGC 2300
group was about twenty times that in the gas and the galaxies. If a similar
ratio applied everywhere, the Universe might be so massive that it would
some day collapse.

But the claim was based on faulty analysis, say Mark Henriksen of the
University of Alabama at Tuscaloosa and Gary Mamon of the Observatoire de
Paris-Meudon in France. They reached the opposite conclusion after using
more sophisticated models to analyse the same data.

The NGC 2300 group, which is between 80 and 190 million light years
away, includes at least four galaxies that are gravitationally bound to
each other. It resembles the Local Group, the collection of galaxies which
includes our Galaxy.

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The gas pervading the NGC 2300 group is so hot that it emits X-rays,
which is how the gas was discovered in the first place. The high temperature
means that the ions and electrons in the gas move fast; the NGC 2300 group
must therefore have a gravitational pull strong enough to keep them from
escaping. By calculating the gravity needed, astronomers can estimate the
group’s total mass.

Henriksen and Mamon found that the total mass in the group is only half
that originally claimed. They also estimate that the mass of gas is four
times as great as had been claimed. From these numbers, and the amount of
mass in the four galaxies, Henriksen and Mamon conclude that the group’s
total mass is only four times that of the galaxies and gas.

If a similar number applies everywhere, the Universe contains insufficient
mass to stop its expansion. The density needed depends on the square of
the Hubble constant. The value of the Hubble constant, which is disputed,
measures the present rate of expansion of the Universe. The larger the Hubble
constant and the faster the expansion of the Universe, the greater the density
required to halt the expansion. So, for a given density, the larger the
Hubble constant, the more the Universe falls short of the density needed
to halt the expansion.

The value of the Hubble constant is probably somewhere between 40 and
90 kilometres per second per megaparsec (1 megaparsec is 3.26 million light
years). For this range of values, Henriksen and Mamon calculate that the
Universe has only 10 to 50 per cent of the mass needed to halt the expansion,
if the NGC 2300 group is representative of the Universe.

This estimate for the density of the Universe is in line with those
inferred from two other galaxy groups in which gas that emits X-rays has
recently been discovered. It also agrees with estimates that have been made
for galaxy clusters, which are more massive than groups and contain hundreds
or thousands of galaxies.

Henriksen and Mamon will report their work in the 1 February 1994 issue
of Astrophysical Journal Letters.